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But that's not compatible with Bruno's idea of eliminating the
physical - at least not unless he can solve the basis problem.

Could you do me a favor and explain what the basis problem is in a
way that a 6th grader could understand? I've found all kinds of
things said on it, and they all seem to be asking different things.

For physicists, it's part of the problem of explaining the
emergence of the classical world from the quantum world.
Decoherence can diagonalize (approximately) a reduced density
matrix IN SOME BASIS.

Is this the same basis as in "momentum basis" and "position basis",
or is it some other usage of the term?

Forgive my ignorance, but what does it mean to "diagonalize a
reduced density matrix"?

It means to take an average over all the other variables except
those of interest (i.e. the ones you measure). If you do this in a
particular basis we think it makes the submatrix corresponding to
those variables diagonal. Then it can be interpreted as the
probabilities of the different values. Note that it is a
mathematical operation that depends on choosing a basis, not a
physical process. The MWI view is that this is a physical process
- which it could be IF the basis was not an arbitrary choice but was
somehow dictated by the physics. But so far there are only hand
waving arguments that "it must be that way".

The whole picture does not depend on the basis. Everett made this very
clear, in my opinion.
The choice of the base happened in the earlier apparition of life. As
I said, the first amoeba did it.

Being diagonal in one basis means it's superposition in some
other basis. So for physicists the problem is saying what
privileges or picks out the particular bases we see in
experiments. Why do our instruments have needles
that are in eigen states of position, while some other things (e.g.
atoms) are in eigen states of energy or eigen states of momentum.
For physicists there are some suggestive, but not fully worked out
answers to these questions, e.g. you get position eigenstates
because the interaction term of the Hamiltonian is a function of
position. But those answers assume the physics. If you want to
reconstruct physics from experiences, you can't borrow the physical
explanation to say why your experiences are classical.

I think the assumption that experiences are classical comes from
the classicality of Turing machines (which are the supposed
mechanism by which experiences are manifest).

I don't think there's anything either classical or quantum about
Turing machines. They are just mathematical abstractions.

Those mathematical abstraction relies on the fact that such machine
can distinguish some classical, well defined, states. The tape
contains bits, not qubits, for all practical evolutionnary purpose. Of
course, in their physical implementation, those are qubit, and that is
why the state of the machines differentiate into quasi-classical
world. Why nature did not evolve quantum brain might be explained by
the fact that evolution has favorised classical macroscopic mind, for
reason comparable of why nature did not evolve any wheels.

If you want, the base has been selected by the first macroscopic
organism, or the first Turing universal system. The theory of
decoherence explains well why macroscopic objects leads to classical
states. It is probably as difficult for nature than for humans to
isolate a quantum macroscopic universal system. We are classical
because our bodies contains many atoms entangling quickly their states
with their neighborhood. It is a good thing as this explains how the
quantum reality defined a first person *plural*, duplicating us
together.

Bruno

And assuming they read and write qubits instead of bits doesn't
change the range of things they can compute.

Brent
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